Immunochromatographic strip formats have become increasingly popular for qualitative and semi-quantitative assays which use visual detection schemes. This type of assay involves the application of a liquid test sample suspected of containing an analyte to be detected to an application zone of an immunochromatographic test strip. The strip is comprised of a matrix of absorbent material through which the test fluid and reagents for detecting the analyte can flow by capillarity from the strip's application zone to a capture zone where a detectable signal, or the absence thereof, reveals the presence of the analyte. Typically, the strip will include means for immunospecifically binding the analyte to be detected with its specific binding partner which bears the detectable label. In one such scheme, the strip contains an enzyme-labeled, mobile binding partner for the analyte which is located in a zone downstream from the sample application zone. If analyte is present in the test sample, it will combine with its labeled binding partner to form a complex which will flow along the strip to a detection zone which contains a substrate for the enzyme label which is capable of providing a colored response in the presence of the enzyme. The strip may contain a zone in which the analyte is immobilized, so that a labeled binding partner which does not combine with analyte, due to the absence of analyte in the sample, will be captured and thereby inhibited from reaching the detection zone. There have been various modifications of this technique, all of which involve some competitive specific binding system in which the presence or absence of analyte in the test sample is determined by the detection or lack thereof of labeled binding partner in the detection zone.
An alternative to the above described immunometric assay which detects the free-labeled binding partner is the so-called sandwich format in which the capture zone contains immobilized antibodies against an epitope of the analyte which is different from the epitope to which the labeled antibody is specific. In this format, the analyte is sandwiched between the immobilized and labeled specific binding partners, and it is therefore an immunometric assay which detects the bound, labeled specific binding partner.
Not all of the schemes for immunochromatography rely on an enzyme-labeled binding partner/enzyme substrate for providing the signal for detection of the analyte. For example, U.S. Pat. No. 4,806,311 discloses a multizone test device for the specific binding assay determination of an analyte and an immobilized binding partner therefor together with a capture zone for receiving labeled reagent which migrates thereto from the reagent zone. The capture zone contains an immobilized form of a binding substance for the labeled reagent. The labeled reagent bears a chemical group having a detectable physical property, so that it does not require a chemical reaction with another substance in order to be detected. Exemplary of such groups are species of fluorescers, phosphorescent molecules, radioisotopes and electroactive moieties.
U.S. Pat. No. 4,703,017 describes the use of visible particulate labels for the receptor. Various particulate labels such as gold sol particles and visible dye containing liposomes are mentioned. PCT Patent Application Publication No. WO 96/34271 discloses a device for determining a target analyte and creatinine in a fluid test sample. The device has an assay strip for the detection of creatinine and a second assay strip for the detection of the target analyte. The creatinine concentration can be determined colorimetrically or by the specific capture of labeled creatinine binding partners. The concentration of the target analyte is corrected based on the sample's creatinine concentration which correction can either be done manually or by means of a properly programmed reflectance analyzer.
Immunochromatographic strip formats provide a viable system for the determination of various analytes (whether they be antigens or antibodies) but suffer from the limitation that they yield results which are at best semi-quantitative when, for some analytes, more precise, quantitative results are required. The strip can be prepared from any matrix material through which the test fluid carrying the analyte and labeled binder-analyte contained therein can flow by capillarity; the matrix can be of a material which is capable of supporting non-bibulous lateral flow, described in U.S. Pat. No. 4,943,522 as liquid flow in which all of the dissolved or dispersed components of the liquid are carried through the matrix at substantially equal rates and with relatively unimpaired flow as contrasted to preferential retention of one or more components as would be the case if the matrix material were capable of absorbing or imbibing one or more of the components. An example of such matrix material is the high density or ultra high molecular weight polyethylene sheet material from Porex Technologies of Fairburn, Ga. Equally suitable for use as the matrix from which the chromatographic strip can be fabricated are bibulous materials such as paper, nitrocellulose, and nylon.
One variable which needs to be controlled in analyses using immunochromatographic strips is temperature control. Temperature is an important variable because all immunochemical reactions are characterized by two temperature-dependent opposite reactions at the same time. These are the formation of an immune complex from an antigen and its antibody and the appearance of free antigen and antibody by dissociation of the immuno complex. Increasing the temperature increases the rate of reaction, and because immunochromatographic strip formats are usually measured under nonequilibrium conditions due to the short assay times involved, temperature control, both within and between laboratories, is critical for insuring consistent reaction rates thereby providing more reproducible assay quantitation. Currently, temperature is not controlled. Typically immunochromatographic strips are run at ambient temperatures which can range from 20-30° Centigrade. Because reaction rates approximately double for every 10 degree centigrade increase in temperature, it is apparent that controlling temperature allows for control of the immunochemical reaction thereby leading to more reproducible results.
Devices for controlling the temperature of an assay include a disposable test cassette having a thermally conductive member, such as an aluminum bar, disposed within the cassette. In such a device, the thermally conductive bar is in thermal communication with the test strip for regulating the temperature of the assay. The thermally conductive bar accelerates the warning of the assay during an incubation period. After the assay is completed, the cassette including the thermally conductive member is discarded. These devices have achieved good results in controlling the temperature of an assay. However, one drawback associated with such a device is that the thermally conductive member is discarded along with the disposable test cassette having the thermally conductive member disposed therein. Therefore, the costs associated with providing the thermally conductive member are incurred with each use of the disposable test cassette having a thermally conductive member disposed therein. Accordingly, there is a need for a low-cost, easily manufacturable test cassette for controlling the temperature of an assay.